[Feature] Extended Antigravity support: MCP, hooks, workflows, sub-agents

[Minitour]

Description

Follow-up to #241. That issue moved Antigravity skills to .agents/skills, but as @MRDGH2821 pointed out, Antigravity also supports MCP, hooks, and several other customization types. Right now capa only wires up skills, AGENTS.md, and rules for the antigravity provider (src/shared/providers/entries/partial-integration.ts). This issue covers the rest.

Current state

Feature	Status in capa
Skills (.agents/skills/)	✅ Done (#241)
Instructions (AGENTS.md)	✅ Done
Rules (.agents/rules/*.md)	✅ Done
MCP (.agents/mcp_config.json)	❌ Held back: remote entries need serverUrl, not url
Hooks (.agents/hooks.json)	❌ An antigravity hook shape exists in hook-handlers.ts, but the provider entry has no hooks config
Workflows / slash commands	❌ Not wired up
Sub-agents	❌ Not wired up
Plugins	❌ Not declared

Scope

• MCP: add entryUrlKey: 'serverUrl' support to McpIntegration (if it isn't there yet), then register capa in .agents/mcp_config.json (global: ~/.gemini/config/mcp_config.json). Check that custom headers work.
• Hooks: add a hooks integration for .agents/hooks.json (global: ~/.gemini/config/hooks.json). Docs list the events PreToolUse, PostToolUse, PreInvocation, PostInvocation, and Stop. Matchers are regexes (""/"*" match everything) and only apply to the tool events. Check the top-level shape: the docs say the file maps hook names to event configs, which may not match the Claude-like shape buildClaudeLikeEntry currently assumes.
• Workflows: map capa commands/prompts to Antigravity workflows (see Rules & workflows).
• Sub-agents: add a subagents integration if Antigravity has a file-based format.
• Plugins / sidecars: find out whether capa plugin manifests map onto these, or leave them out and say why in the docs.
• Detection: detectInstalled checks the legacy .agent/ dir. It should also check .agents/.
• Update docs/providers/antigravity.md, the matrix in docs/providers/README.md, and registry.test.ts.
• Consider moving antigravity out of partial-integration.ts into its own entry file once it's fully supported.

References

• MCP: https://antigravity.google/docs/mcp/
• Hooks: https://antigravity.google/docs/hooks/
• Skills: https://antigravity.google/docs/skills/
• Rules & workflows: https://antigravity.google/docs/rules-workflows/